The disclosure is based on a method for determination of the state of charge of a battery pack which consists of a plurality of individual battery cells. The subject matter of the present disclosure also covers a battery system consisting of a lithium-ion battery pack and a battery management system which operates on the basis of the above method, as well as a motor vehicle which contains a battery system consisting of a lithium-ion battery pack and a corresponding battery management system. The subject matter of the disclosure furthermore covers a computer program, which carries out all the steps of the method according to the disclosure when it is run on a computation appliance.
In many applications, for example in hybrid or electric vehicles, electrical energy which is stored in a battery is used as an energy source. It is critical for the operation of a battery such as this in this case to be able to determine the state that the battery is in and the capacity that it has for further operation. For this purpose, the operation of the battery is managed by means of a battery management system BMS. Inter alia, this battery management system determines the state of charge SOC for each individual cell. The state of charge may in this case vary between complete discharge and complete charge.
Battery packs based on lithium-ion or NiMH technology are used in hybrid and electric vehicles, consisting of a large number of electrochemical cells connected in series and/or in parallel. The battery management system is used to monitor the battery and, in addition to safety monitoring, is also intended to ensure as long a life as possible.
The states of charge of the individual battery cells within the battery pack may in some circumstances differ considerably. However, the indication of a single, valid and representative state of charge value is necessary for superordinate controllers or for indicating the state of charge to the user.
It is known from the prior art for the mean value of the states of charge of the individual battery cells to be output. This is associated with the problem that individual cells could be overcharged if the mean state of charge is still below 100%, that is to say below the maximum state of charge of complete charge. Individual cells can likewise be undercharged if the mean SOC is still above 0%, that is to say above the minimum state of charge for complete discharge. One disadvantage in this case is that undercharging or overcharging of electrochemical battery cells can lead to accelerated aging or else to damage to the battery cells. In the extreme case, this may result in a battery fire or explosion.
It is also known for the maximum and minimum value of the individual battery cells to be specified. Overcharging and uncharging can be avoided by evaluation of the two values. However, the handling of two values is associated with complications, for example in the indication of the total state of charge or in the indication or determination of the remaining life or of the available capacity. Furthermore, sudden changes when changing from one value to the other are unavoidable.
Furthermore, it is known for the maximum value of the individual battery cells to be quoted for high states of charge, and for the minimum value of the individual battery cells to be quoted for low states of charge. However, one disadvantage in this case is that the battery pack state of charge changes suddenly and discontinuously at the threshold value, which can lead to a behavior which cannot be calculated, to a behavior which cannot be understood, or to sudden changes in state of charge indications.